If you have worked on transmissions for combustion engine vehicles, many transmissions on HEV and PHEV will be familiar. Some need more explaining. Most techs know about the Toyota power split device or eCVT, but did you know that Ford uses the same principle? What about the Chevy Volt? Let us take a closer look. Those transmissions require more study so we will do what we can in this article. Other makes, like Honda, have more unique designs. At ACDC we call all high voltage vehicles “EMVs.” There are great videos that animate the working of an EMV transmission, so use them to supplement this article. The added instant torque of high voltage motors has stressed many conventional transmissions, like the VW Jetta HEV. There is a lot to learn, so let’s get started.
The conventional transmission was originally designed for a vehicle with an internal combustion engine (ICE). As the vehicle gained road speed, the RPMs of the crankshaft could be slowed down by shifting to a higher gear ratio and then the “torque” would be in a RPM range that had the ability to keep accelerating the vehicle. Once a powerful electric motor was added to the driveline, the need for more torque was reduced. As long as an ICE was used, a transmission was needed. The road speed needs to match the torque of the ICE. When the ICE was eliminated, the changing of gears became optional. Most EV applications are single speed gear boxes. The most common set up is a range of 7 to 10 to 1 gear ratio. No shifting with direct drive. Some transmissions have a high voltage motor(s) located inside (Figure 1) while other designs locate the motor in between the transmission and ICE. As long as the wheels turn the right direction when they need to, anything is open for development.
Standard Shift Transmission
Honda was the only OEM to incorporate this type of transmission into a light duty hybrid car. The Honda Insight (Figure 2) came out in M/Y 2000 and a 5-speed manual transmission was all they offered. To make this happen, two switches were installed in the transmission, one for first gear and one for neutral. A clutch switch was also attached, so when the car was slowing down to 18 mph, the ICE would shut off and the car would coast to a stop. The driver had to be trained to keep the transmission in neutral or the ICE would continue to run. If the clutch was out and the car was in “N”, the ICE would stay off for a long time. When the driver put the clutch to the floor and shifted into first gear, the ICE would start instantly and silently, if it was in “READY to DRIVE” mode. Later on the Honda CR-Z HEV could be ordered with six-speed stick.
CVT and eCVT
What is the difference between a “CVT” and an “eCVT”? A CVT (Fig 3) is a Constantly Variable Transmission. The difference mechanically speaking is an eCVT uses two electric motors, the ICE and a planetary gear set to maintain ideal engine rpm and wheel speed. A CVT uses a metal belt and two hydraulically actuated pulleys to maintain optimal engine rpm to wheel speed. The CVT and eCVT allow the ICE to run at an rpm suitable to power the car’s wheels independently of the vehicle’s speed. Because of this, the ICE doesn’t have to increase in speed as the car goes faster, so the car feels very different than a “geared” automatic or standard shift vehicle. The torque multiplication effect of a CVT is absent in an eCVT and is replaced by the torque of the electric motor assisting the ICE.
If you do not already know about the eCVT, here is a brief overview on the eCVT used in most Ford and Toyota HEVs and PHEVs. The engine’s crankshaft is connected to the planet carrier (Fig 4). Motor/Generator 1 (M/G1) is connected to the sun gear and M/G2 is connected to the ring gear. The rotation of the ring gear drives the car via reduction gears or a chain and then to the differential. If you already know that, make sure you keep the terms CVT and eCVT straight as you study more on your own. This can seem like a complex subject, so make sure you study and learn from someone with the knowledge needed to help you learn more. You need to get clear about the Toyota/Ford eCVT to understand the power flowing to the wheels.
Toyota / Ford Modes of Operation
Here are the modes that most Toyota hybrids use (generation 1998 to 2024) and most Fords use (2005 to 2024). On the 2016 Prius liftback (Gen 4), mode 10 was added. If you know these modes then your skill level is high and troubleshooting will be easier for you. We do not have space here to explain each mode, so get more training if you need it.
- Starting the ICE with M/G1. Must be in park.
- Backing up. Where is reverse gear?
- Slow speed electric drive on HV battery only.
- Reverse with ICE on. Why?
- Starting the ICE while driving in EV mode.
- Driving with the ICE, the CVT function.
- Full Power operation.
- Regenerative Braking in “D”.
- Braking in “B” mode. (Ford uses the letter “L”)
- Full Power — 2016-2024 Prius has a twist.
Another Dutchman, Niels Blaauw, put a video together and in under 15 minutes you will get an animated view of the inner workings of a Power Split Device (PSD) in a hybrid.
AWD Systems
In order for a FWD hybrid to be an AWD vehicle, Toyota designed and built a rear drive differential (Figure 5) that was powered by a three-phase motor. Toyota called it Motor/Generator Rear (M/GR). Ford chose to add a “Power Take Off” (PTO) to their eCVT and add a conventional drive shaft and typical rear differential drive in their Ford Escape (M/Y 2005-2012). The Ford Escape hybrid came in FWD or AWD. The PTO was bolted to the eCVT so the same eCVT was used in all drive types. Understanding the Ford AWD system is conventional training so we will not cover that here.
Toyota MGR
When the Lexus RX 400h and the Toyota Highlander HEV came to America in M/Y 2006, they had AWD. It was an option on the Toyota. No drive shaft — just three orange cables heading from the inverter to the rear axle. This system is very common today.
The Volt Transmission
The M/Y 2011-2015 drive unit is a 4ET50 (Fig 6). There are two electric motors, one traction motor/generator, M/G2 (GM calls it motor B) rated at 111kW and the other motor generator, M/G1 (or motor A in GM terms), rated at 58kW, a 1.4 liter gasoline engine, one planetary gearset to blend power inputs, and three clutches to control power inputs.
The Drive Modes
Mode 1: When the battery pack is charged, M/G2 is the sole source of propulsion at low speeds and hard acceleration from a stop. To do this, the planetary ring gear is locked to the case with clutch 1 (C1) and M/G2 is rotating the sun gear. The planet gears are spinning and forcing the carrier to move (Figure 7). The carrier is geared to the final drive and the wheels are moved in either a forward or rearward direction.
Mode 2: This is now a two motor EV. As speed increases (about 40 mph), the ring gear is unlocked by opening C1 and coupled to M/G1 with C2, allowing both motors to work in tandem for improved efficiency and adding more range of pure EV operation at highway speeds. Watching the PIDs for M/G2 and M/G1 while someone else drives will help you see the inner workings of a great design. Both motors will be spinning in the same direction but at lower speeds than in Mode 1. This is why the range is slightly better than a single motor drive system.
Mode 3: When the battery pack reaches its minimum charge of about 20% in normal operation, or 40% in Mountain Mode, the Volt goes back to Mode 1. Based on load and SOC, the ICE will start when C3 is closed and M/G1 cranks the ICE. Under hard acceleration, electricity generated by the 1.4L ICE, plus reserves in the battery pack, provide Mode 1 operation. Additional electricity from M/G1, being rotated by the ICE, helps maintain a minimum charge in the battery pack (about 20% SOC). Once you are at a charging station, plug into a charger (really an EVSE) and recharge the HV pack. The reasoning here is not to fully charge the HV battery as the fuel economy would be very low. The Volt will shift out of Mode 3 if the car will be more fuel efficient. That happens at about 40 mph. In mode 3, the ICE is only a generator.
Mode 4: The Volt is now a hybrid with two motors. The operation is similar to Mode 3, but now the ICE is started by M/G1 with C3 closed, connected to the ring gear via C2. By connecting the clutches, some torque from the gasoline engine is transferred through M/G1 (a generator now) to the ring gear and from there to the wheels. M/G1, via the battery pack and inverters, provides some electricity to M/G2. M/G2 and the ICE are the sources of propulsion. M/G2 must have power or the eCVT will be in neutral. Blending torque from M/G2 (the traction motor) and the ICE increases efficiency at highway speeds by 10 to 15%, rather than the use of the M/G2 alone. Mode 4 uses the same principle as the Toyota Prius eCVT. Less fuel is being used at higher road speeds than in Mode 3.
AWD Tesla System
All Tesla cars were RWD for years. The inverter, drive system and three-phase motor (Fig 8) were housed in one unit. The transmission was not used for shifting, but it did multiply torque. The single gear ratio was chosen by knowing the torque, the maximum rpm of the motor and the top road speed required. Tesla announced the introduction of All-Wheel Drive (AWD) versions of the Model S (designated by a D at the end of the model number), on April 8, 2015. The Model S 70D was Tesla’s first AWD EV. A smaller unit “Motor/Generator Front” (M/GF) was installed up front. When your foot was at the floor, M/G2 (rear motor) was plenty fast. The front motor (M/GF) added the all-wheel drive function so when you wanted more power or traction (like a snowy day), the AWD system was the answer. Most AWD EVs are using a system like this today. Tesla certainly made electric cars popular.
Summary
The purpose of the transmission, when combined with an internal combustion engine, had a clear purpose — to multiply torque and go faster. The Prius transmission did that and more. Lots more. It had to blend two power sources. The Volt was a unique system and was an EV or a Prius, depending on the SOC of the high voltage pack. The Honda two-motor system used today is not a CVT, although it is named that. The world of shifting is going away, slowly, but steadily. With more EVs on the road than ever before, the transmission as we know it may be gone at some point in light duty vehicles. No matter what happens next, you must learn it and adapt, or you will be left behind.
